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$4                            DR.   STANFORD'S   REALMS                         CHAR
can be traced, it results as a conclusion that from a given area of
origin the group in question migrated in all directions where
possible to a varying degree; it then died out in intervening
tracts, or was left only In a certain part of its former and more
extensive area of range.
Zoological Reg-ions.—Seeing that each species of animal has
its own definite range, it is clear that the earth's surface can be
apportioned into divisions which are characterised by their
animal inhabitants. We shall divide the earth into realms,
which are the largest divisions; then into regions; and
finally into subregions. It must be borne in mind that the
various groups of the animal kingdom are of different ages,
geologically speaking, and have therefore had less or more time,
as the case may be, to settle down into their present distribution,
and that different animals differ greatly in their rate of multi-
plication, their power of migration, and their susceptibility
to the effectiveness of various natural and other barriers to
distribution. It is not, therefore, possible to divide the
world into realms and regions which shall express the facts of
distribution of the entire animal kingdom. Such divisions,
which are common in text-books of zoology having but a small
section devoted to zoogeography, are at best mere approximations
and averages; no good is gained by taking such a comprehen-
sive view of the matter, as the essential object of subdividing
the earth's surface is thereby lost sight of. The isoogeographical
division of the earth which will be adopted here is that origin-
ally recommended by Dr. Blanford, and now accepted by a
number of authorities. There are three " realms,** to which a
fourth may perhaps be added—though on negative grounds, and
merely for the purpose of emphasising the parts of the world to
which mammals have not gained access. The realms are again
divisible into regions, at least in the case of one of them, and
the regions may be again separated into more or less distinct
subregions or provinces. The three primary divisions or realms
which contain mammals are the JSTotogaean, including Australia
and certain islands to the north of it; the Keogaean, or the
South American continent and Central America; the Arcto-
gaean, including the continents of North America, Europe, Asia,
and Africa, together with the adjacent islands, such as the
t,. Indies, Bast Indies (exclusive of those whieh fall within